function phi = phi3(spectrum,m,n,x,y)
LatticeSize = spectrum.LatticeSize;
XYmax = spectrum.XYmax;
if (n==0||2*n==LatticeSize)
    coeff = sqrt(2)/XYmax;
else
    coeff = 2.0/XYmax;
end
phi = coeff*sin(2*m*pi/LatticeSize*x)*cos(2*n*pi/LatticeSize);